1. Technical Field
The present invention relates in general to an improved hard disk drive design and, in particular, to an improved method of erasing or preconditioning the magnetic media of the disk or disks in a hard disk drive at the assembly level.
2. Description of the Related Art
Generally, a data access and storage system consists of one or more storage devices that store data on magnetic storage media. For example, a magnetic storage device is known as a direct access storage device (DASD) or a hard disk drive (HDD) and includes one or more disks, read/write head(s) with actuator and associated controller and electronics to manage local operations concerning the actuator, disks and data stored on the disks. The hard disk(s) themselves are usually made of a substrate of aluminum alloy or a mixture of glass and ceramic, and are covered with a magnetic film coating. The magnetic coating can be several layers of different metal alloys. Typically, the one or more disks are coaxially positioned on a common spindle that is turned by a disk drive motor at several thousand revolutions per minute (rpm).
The disk(s), motorized spindle, read/write heads and actuator are all enclosed using a base and top cover. This assembly level is typically called a head disk enclosure (HDE). The HDE is sealed to maintain an exceptionally clean environment for the read/write heads and recording media. The read/write head, connected to a suspension and arm assembly, is linearly or pivotally moved utilizing a magnet/coil structure that is often called a voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which the spindle is also mounted. The base casting with its spindle, actuator VCM, and internal filtration system is then enclosed with a cover and seal assembly to ensure that no contaminants can enter and adversely affect the reliability of the read/write head or disk media.
A motor, typically a brushless DC motor, is used to rotate the disk. The disk is mounted and clamped to a hub of the motor. The hub provides a disk mounting surface and a means to attach an additional part or parts to clamp the disk to the hub. In most typical motor configurations of HDDs (hard disk drives), the rotating part of the motor (the rotor) is attached to or is an integral part of the hub. The rotor consists of a ring-shaped magnet with alternating magnetic north/south poles arranged radially and a ferrous metal backing. The magnet interacts with the motor's stator by means of magnetic forces. Magnetic fields and resulting magnetic forces are induced via the electric current in the coiled wire of the motor stator. The ferrous metal backing of the rotor acts as a magnetic return path. For smooth and proper operation of the motor, the rotor magnet magnetic pole pattern should not be substantially altered after it is magnetically charged during the motor's manufacturing process.
The disk recording media typically has several layers that make up a compound magnetic structure. The structure typically includes a high moment soft underlayer and a magnetic top layers which may magnetically couple and help achieve improved magnetic stability, low noise and increased coercivity. The disk magnetic media may be a plurality of compositions and layered designs and could be either longitudinal or perpendicular in orientation of magnetic domains. An external magnetic field can be applied to the disk for preconditioning by orienting the media or erasing any servo pattern data. The required direction of the magnetic field is dependent on the magnetic film media design.
A process called “disk erase” is often a necessary part of manufacturing an HDD. Disk erase involves preconditioning the media by orienting the magnetic domains of the disk's magnetic layer in a consistent uniform state. This process can initially be done prior to HDD assembly as part of the disk manufacturing process, or ideally, while the disk is in the HDE. In addition to initial HDD assembly, disk erase is needed for HDDs that have failed the HDD test or servo write process and have to be reworked by repeating the servo write process. In such cases it would be prohibitively expensive to remove the disk and perform a disk level erase. It is most cost effective to “erase” the disk(s) while they are in the HDE. This can be achieved by one of several methods: 1. use the HDD product read/write heads to write a fixed and uniform pattern over the disk thereby erasing any prior servo pattern. 2. Use a special write head or magnet which enters into the HDE through a port or window in the top cover or base casting. Such port or window would be sealed during normal HDD operation. 3. Expose a portion of the HDE which houses the disk to a high intensity magnetic field while spinning the disk(s).
The method that uses the product heard to erase the disk track by track takes significant time because the head must erase each individual track, one at a time, and progressively move from track to track. Also, the process using the product head is typically done at the servo track write (STW) process, which is currently the single most expensive HDD manufacturing assembly and/or test process due primarily to the cost of the equipment and its relatively limited production capacity throughput.
A method that uses a special manufacturing process write head or a bar magnet brought close to the surface of the disk to erase the disk can have some advantages over using the product head. Such a process can be faster but a disadvantage of this process is that it requires a hole or access port in HDE to allow entry of the special process head or magnet. In addition, this process would have to be performed in a clean environment to prevent contaminants from entering the HDE through the magnet access port.
The third method, exposing a portion of the HDE that houses the disk to a high intensity magnetic field via an HDE level disk erase apparatus, is most desirable for several reasons: 1) it does not require opening up the HDE which would require time and expense and exposes the HDE to possible contamination, and 2) it is fastest and can be performed on fairly simple inexpensive equipment. This method does require that the materials of the top cover and base be non-magnetic. Such use of non-magnetic materials for top cover and base is current state of the art, usual practice.
Although HDE level disk erase is most advantageous, there are several limiting problems:    1. Increasing disk coercivity makes it increasingly difficult to erase the disk    2. The innermost area of the disk is difficult to erase without magnetically damaging the motor rotor    3. Eddy current effects when using aluminum disk substrates make it difficult to control spinning the disks.
With successive HDD products of increased areal density the disk media coercivity is increasing. The increased, coercivity of the disk increases the resistance to demagnetization (i.e., changing the state of the magnetic domains), thus requiring an increased demagnetizing force to erase the disk (write to the disk). Increasing the magnetic field strength of an HDD level disk erase machine exacerbates the problem of modifying, i.e., damaging, (via demagnetization) the spindle motor rotor. Thus, a method is needed to: (1) increase the magnetic field strength of an HDD level disk eraser, and (2) increase the magnetic field without increasing the magnetic field near the spindle motor rotor and/or a method of protecting the spindle motor rotor magnet.
With perpendicular recording media the required magnetic field direction of an HDE level disk erase apparatus is different than that typically used for longitudinal recording media. The required direction is dependent on the design of the compound layers of the media and can be perpendicular to the disk. For longitudinal recording the usual required direction is tangential with respect to the disk.
With increasing magnetic data areal density there is a trend towards an increase in the proportion of HDDs having only one disk. For HDDs having only one disk, it is possible to have an enclosure design, which provides a minimum distance between the disk surface and the outer surface of the enclosure. Such a design could allow for the magnets of a disk erase apparatus to be much closer to the disk and allow for an improved disk erase machine.